<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .box {
            width: 200px;
            height: 200px;
            border: 1px solid rgb(98, 245, 49);
            margin-top: 100px ;
            position: relative;
        }

        .box::before {
            content: '';
            width: 0;
            height: 0;
            border: 50px;
            border-style: solid;
            border-color: transparent transparent rgb(251, 236, 16) transparent;
            position: absolute;
            top: -50%;
            left: 25.5%;
        }

        .box:after {
            content: '';
            width: 0;
            height: 0;
            border: 50px;
            border-style: solid;
            border-color: transparent transparent white transparent;
            position: absolute;
            top: -49.7%;
            left: 25.5%;
        }
    </style>
</head>

<body>

    <div class="box">

    </div>

</body>

</html>